1. Field of the Invention
This invention relates to a nitride III-V compound semiconductor substrate, its manufacturing method, a method of manufacturing a semiconductor light emitting device and a method of manufacturing a semiconductor device, which are especially suitable for use in manufacturing semiconductor lasers and light emitting diodes, or electron transporting devices.
2. Description of the Related Art
In recent years, semiconductor lasers using nitride III-V compound semiconductors such as AlGaInN (hereinbelow called GaN compound semiconductor lasers) have been under active research and developments in the hope of making semiconductor lasers capable of emitting light over the range from the blue region to the ultraviolet region necessary for enhancing the density of optical discs. Lately, efforts are being expended to further improve their lifetimes and properties toward their practical use.
When manufacturing such a GaN compound semiconductor laser, a laser structure is most typically formed by crystal growth of a GaN compound semiconductor layer on a sapphire substrate. For example, using a sapphire substrate sized 50 mm (2 inches) in diameter and 430 μm in thickness, a GaN compound semiconductor layer is grown thereon up to a thickness around 7 μm in total.
However, if a GaN compound semiconductor has a thickness around 7 μm on a sapphire substrate as mentioned above, the sapphire substrate warps due to a difference in thermal expansion coefficient between the sapphire and the nitride III-V compound semiconductor such as GaN. This warpage measures as large as 80 μm.
This large warpage of the sapphire substrate adversely works against exposure in a manufacturing process of a GaN compound semiconductor laser and polishing of the bottom surface of the sapphire substrate.
More specifically, in the exposure process, the sapphire substrate, having GaN compound semiconductor layers grown thereon and a resist coated on its surface, undergoes exposure through a photo mask. If the sapphire substrate largely warps as mentioned above, distance between the photo mask and the resist may become uneven within the area of the substrate, or a dimensional deviation may be produced between the photo mask and the substrate within the area of the substrate. Thus the mask cannot accurately fit the entire surface of the substrate. As a result, especially when a base GaN layer is laterally grown on the sapphire substrate by ELO (epitaxial lateral overgrowth) and GaN compound semiconductor layers forming a laser structure are grown thereon by crystal growth, it is difficult to form a ridge in a less-defective region (wing portion) between a seed crystal and a coalescing portion of the lateral growth, and the ridge often deviates from the wing portion. Therefore, this problem adversely affects the laser properties, and in particular, its lifetime, and also degrades the production yield.
For making cavity edges, it is the most usual way to cleave a sapphire substrate having GaN compound semiconductor layers grown thereon. For easier cleavage, it is necessary to thin the sapphire substrate by partly removing it from the bottom by polishing. However, if the sapphire substrate largely warps as mentioned above, it often cracks during polishing.
Furthermore, if the warpage of the sapphire substrate is large during crystal growth of the GaN compound semiconductor layers, because of uneven temperature distribution along the plane, the resulting GaN compound semiconductor layers become uneven in composition and thickness.
Under the circumstances, there is a demand for a technique capable of sufficiently reducing warpage of substrates, namely, not to exceed 70 μm, to remove the above-mentioned problems caused by warpage of sapphire substrates.